selftests: xsk: Introduce type for thread function

Introduce a typedef of the thread function so this can be passed to
init_iface() in order to simplify that function.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210907071928.9750-3-magnus.karlsson@gmail.com